A construction machine including an engine has a problem of noise generated in an engine compartment, especially, the noise generated by a fan (cooling fan) in the engine compartment and emitted outwardly from an air intake opening of the engine compartment. For example, noise around the construction machine (i.e., at a construction site, etc.) and noise at the ear in a cab are problematic.
FIG. 1 of Patent Literature 1 shows a technique of mounting sound absorbers in a cooling air intake duct. FIG. 1 of Patent Literature 2 shows a technique of mounting a sound absorber near an air intake opening. These techniques aim to reduce noise while taking in cooling air through the air intake opening.
Besides, there are some cases of using an interference type muffler, such as a Helmholtz resonator or a side branch, to reduce noise. Patent Literatures 3 and 4 describe about reducing noise by use of a side branch.
The sound absorber disclosed in Patent Literatures 1 and 2, however, can suppress no noise but one having a limited frequency. Specifically, the sound absorber has the ability to suppress high frequency sound (i.e., sound at frequencies higher than 200 Hz), but has little to suppress low frequency sound (i.e., from about 100 Hz to about 200 Hz). In addition, in the case of suppressing the low frequency sound by use of only the interference type muffler, such as a Helmholtz resonator or a side branch disclosed in Patent Literatures 3 and 4, the muffler is required to be large. Such a large muffler is hard to dispose in the engine compartment. For example, the aforementioned side branch requires a length of about 0.425 m to 0.85 m in order to reduce noise of 100 Hz to 200 Hz. Additionally, the muffler cannot exert its noise suppression effect beyond a narrow frequency range.